In the history and development of inflators for inflating vehicle occupant protection inflatable restraints (airbags), one of the earliest types of devices tried was a stored gas system. Such a system consists of a pressure vessel containing gas at high pressure, a rupturable wall in the pressure vessel, means for causing the rupturable wall to rupture upon command, flow-directing passageways to conduct the gas to the airbag, and the airbag itself. When the inflator was operated, the rupturable wall was caused to fail, and the high pressure stored gas rapidly flowed out of the pressure vessel, inflating the airbag. In a traditional stored gas inflator, these are the only significant components. In recent years, traditional stored gas airbag systems have not been widely used.
One of the drawbacks which have prevented the wide use of traditional stored gas inflators is the fact that their output varies significantly with ambient temperature. Airbag inflators are typically specified to operate over a wide temperature range such as -40 C. to +90 C, which represents extreme winter conditions and hot summer conditions in bright sunlight.
The output of an inflator is commonly judged as the rise in pressure in a closed receiving tank when an inflator is discharged into it. The receiving tank is typically the same volume or similar volume as the inflated bag. The rise in tank pressure (final pressure in the receiving tank at the end of the transient compared to initial pressure in the receiving tank) is indicative of the amount of energy which the inflator has produced. When an inflator is discharged into an actual airbag, the majority of this output energy (typically 70% to 90%) is used to fill or expand the bag from its folded position of essentially zero volume to its full volume, and then the remaining small minority of the energy (typically 10% to 30%) is used to raise the pressure of the bag above atmospheric after the bag has been filled (pressurization). For sake of discussion herein and for better understanding of the invention, the middle value may be assumed, i.e., the output of the inflator at a baseline condition is 1.0 energy unit and 0.8 energy units are used for filling the bag and 0.2 energy units are used for pressurizing.
It can be further supposed that because of a reason such as dependence of inflator output on ambient temperature, the inflator output under other conditions may increase to 1.55 energy units. The energy required to fill the bag would remain at 0.8 energy units, but then the energy left over to pressurize the bag would then be 0.75 energy units, an amount which is almost four times what it was in the baseline case. This would significantly increase the peak overpressure (peak pressure in the bag above atmospheric pressure) in the bag, although given the fact that the bag has vents which were not considered in this discussion, the increase would be less than the factor of nearly four. Still, this illustrates that the peak overpressure in the bag is quite sensitive to variations in inflator output. In this example an increase of 55% in inflator output energy resulted in an increase in bag peak overpressure of much more than 55%. The peak overpressure in the bag is an important parameter both because of strength limitations of the bag material and because the performance of the bag in absorbing the occupant's kinetic energy is influenced by the peak bag pressure.
With this in mind, it can be understood why the output of a traditional stored gas inflator varies undesirably with temperature. In absolute terms the temperature range just discussed, for which the inflator is specified to operate, is 233 K. to 363 K., and the ratio of these absolute temperatures is 1.55. Assuming an ideal gas and neglecting any amount of gas which remains inside the inflator after the inflation, the output energy of a traditional stored gas inflator should vary roughly in proportion to the initial absolute temperature of the gas. For most real gases (particularly nitrogen and argon, which are most commonly used), the actual ratio of hot output to cold output for this temperature range is even larger than this number for an ideal gas, due to thermodynamic nonidealities of these gases. With such a large variation, an inflator which filled the bag completely at the lower extreme temperature would be at risk of bursting or tearing the bag open at hot temperature, or at least it would not produce good characteristics for absorbing the impact of a vehicle occupant. In the other way of looking at it, if the inflator or bag size were adjusted so that the bag had proper characteristics at the upper extreme temperature, then at cold initial temperature the inflator would only fill the bag to a portion of its full volume. Neither of these situations is acceptable.
The inflator technologies which are in widespread use today are less sensitive to temperature than the traditional stored gas situation just described, but they do have dependence on temperature. Pure pyrotechnic inflators produce their gas from the combustion of solid chemicals. The combustion process is described by a burn rate which is dependent on the initial temperature of the pyrotechnic material, and it is possible that completeness of combustion may also be affected by initial temperature. Another inflator technology is hybrid inflators, which combine stored gas and solid pyrotechnic chemicals. In a hybrid inflator, the temperature dependence of the stored gas portion of the output is roughly as just described for traditional stored gas inflators, but the portion of the output obtained from pyrotechnic chemicals has less variation with temperature, and so the overall performance of this type of inflator shows less variation with initial temperature than does a pure stored gas inflator. Nevertheless, it can be said that for all of these inflator technologies, temperature dependence of the output is still an issue.
If the temperature dependence of the stored gas inflator could be improved, it would offer an inflator which has significant advantages with respect to several issues that are of increasing importance in the automotive industry. Increasing attention is currently being paid to the amounts of solid particulate and of potentially irritating gaseous substances which are contained in the output gas from inflators utilizing solid pyrotechnic chemicals. This concern is for the comfort, convenience and health of the vehicle occupants when the airbag is used. An improved stored gas inflator would be free of these particulate and objectionable gaseous emissions except perhaps for a minimal amount of emissions resulting from a pyrotechnic initiator which might be used at the rupture disc sealing off the stored gas vessel to initiate rupture of the rupture disc. Also, with the manufacture of airbag inflators now amounting to many millions of units per year, it becomes more apparent that the toxicity of some pyrotechnic substances, particularly sodium azide, is a problem. The toxicity is a problem both during manufacturing and for eventual disposal or recycling of used cars. An improved stored gas inflator would solve all of these health and environmental problems.
In addition, the improved stored gas inflator described here is relatively simple compared to most other inflators, and this has beneficial implications for reliability and for reduction of cost. For any new vehicle with its unique crash characteristics and unique dimensional requirements for the inflator, there is effort and expense involved in developing a vehicle-specific inflator, and a significant part of this effort and expense is related to filtration. Temperature compensated stored gas technology essentially eliminates the whole topic of filtration from the inflator development process, except possibly for a very small extent of filtration related to the rupture disc initiation means if that initiation means is pyrotechnic in nature.